Modulation system



'Feb. 19, 1946.

M. CAWEIN MODULATION SYSTEM I Filed Feb. 14, 1944 INVENTQR hank-JO wnmw mm MADISON, GAWEIN ATTORNEY Patented at. 19, 1946 Madison Cawein, Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation,

a corporation of Delaware Application February 14, 1944, Serial No. 522,297

3 Claims. I

I This invention relates to modulating systems and particularly to systems ofthis character employing electron multipliers.

It has been customary to effect. a signal modulation of a carrier wave by exciting the input circuit of an electron multiplier with radio frequency energy. It then becomes necessary for the multiplier to function at the carrier frequency. Quite satisfactory results have been obtained by the use of apparatus operating in such a-manner so long as the frequency of the carrier wave does not exceed the cutoff frequency of the multiplier.

As is well known in the art, however, an'elec- 'tron multiplier has a definite cutoff frequency.

Apparently, in a multistage electron multiplier the cutoff frequency characteristic is a cumulative effect. With respect to any one secondary emissive electrodethe frequency cutoff characteristic may not be particularly pronounced and, therefore, the energy may not be materially reduced. However,-since each electrode has the same characteristic,there is effected by successive electrodes a number of energy reductions with the result that the carrier frequency energy derived from the output circuit of the device is insufllcient for subsequent use. This condition is particularly prevalent when attempting to operate at relatively high frequencies. However, the condition undoubtedly is present when operating at relatively low carrier frequencies, but to a somewhat lesser degree. Consequently, whereit is desired to eflecta signal modulation of a carrier .wave having a frequency exceeding the multiplier cutoff frequency the input circuit excitation method can not be used.

It, therefore, is an object of the present :invention to provide a modulating system includin an electron multiplier which is capable of operating at relatively high frequencies.

Iu=accordance with the invention, there is pro- I vided an electron multiplier having a secondary electron emissive electrode and an electron collecting electrode and for which there is-provided a source of electrons varying in intensity in accordance with intelligence signals. The electron source and the electrodes are maintained at suit,-

able normal unidirectional potentials with respect to one another. One of these-potentials is modifled at a carrier frequency in a manner suitable to vary the gain of the multiplier between maximum and minimum values at the periodicity of the carrier frequency. The output circuit derived from the collecting electrode includes a network tunedfor resonance at the frequency of the carrier wave, whereby to develop carrier wave energy which is modulated in amplitude in accordance with the intelligence signals represented by the electron admission to the multiplier.

Also, apparatus in accordance with the instant 5 invention may include an amplifier coupled to the output circuit of the multiplier. In this case the biasing potential for the input circuit of the amplifier may be modified in accordance with auxiliary system control signals such as synchronizing impulses. In this manner there i developed in the output circuit of the amplifier a carrier wave which is modulated in amplitude in accordance with a composite signal which in a television'sys tem may include video, blanking and synchronizing components.

gether "with other and further objects thereof. reference is had to' the following description, taken in connection with the accompanying draw- 5 ing, and its scope will be pointed out in the appended claims. '1

The single figure of the drawing is a schematic representation of a circuit embodying the invention.

shown apparatus embodying the invention which is employed in a television transmitting system.

An image analyzing tube of the dissector type is employed todevelop the intelligence signals by which it is desired to modulate the carrier wave. The dissector is shown in a diagrammatic form since it serves merely as a disclosure of a signal generato and forms no part of the-instant invention.

The dissector is provided with a photoelectric cathode l which has the property of emitting a .stream of electrons representative of an optical imag which'is. projected onto the cathode in any well known manner. Opposed to the cathode is the metallic tubular shielding anode 2 which is provided with a small recessed portion I substantially centrally disposed with respect to the cathode l. The recessed portion of the shield is provided with a primary scanning aperture 4. The electron stream emitted by the cathode l is focused by wellknown means (not shown). substantially in the plane of the aperture 4. Deflecanode 2 substantially in'the-iilustrated manner} For a better understanding of the invention, to-

Having reference now to the drawing, there is V rectified power supply. a

The interior surfaces of these electrodes are provided with a substance which has the property of emitting multiplied numbers of electrons when impinged atsuitable velocity by electrons directed thereto. The first stage multiplier electrode 9 is provided with a secondary scanning aperture I3 which preferably is of substantially the same size as an elemental scanning area and is in alignment with the primary scanning aperture 4. The electron multiplying apparatus within the shielding anode also includes an electron collecting electrode l4 which may be in the form of a plate, as shown. The collecting electrode is located adjacent the outlet of the last multiplier electrode l2.

The voltages required. to operate the dissector and multiplying apparatus are derived from a voltage divider connected across the terminals of a source of undirectional energy such as a battery It. Thevoltage divider comprises a series arrangement of a plurality of resistors l8 to 28, inclusive. Also included in the series arrangement is an inductor 21. The divider resistor 28 and the inductor 21 are shunted, respectively, by condensers 28 and 29 for a purpose to be described. One terminal of the resistor I8 is connected to the negative terminal of the battery [5 and to the dissector cathode I. The unidirectional electron accelerating potential necessary to efiect the electron transit between the cathode] and the anode 2 is developed across the resistor l6 and is impressed upon the anodev by a connection from the junction point between the divider resistors i8 and I]. The first stage multiplier electrode 8 is operated at the same potential as the shielding anode 2 to which it is connected by any convenient means such as a conductor 3 I.

The multiplier collecting electrode I4 is connected through a tuned circuit 32 to ground.

The network 32 comprises the parallel connection of an inductor 33 and a condenser 34. The eleventh stage multiplier electrode l2-is maintained at a negative potential with respect to the collecting electrode [4 and ground by means of a source of unidirectional energy such as a battery 38, the positive terminal of which is connected to ground and the negative terminal to the electrode l2. This battery is by-passed for alternating currents by a shunt condenser 36 and also may be shunted by a resistor 31 in the case that the illustrated battery represents a For the purpose of developing carrier frequency energy there is provided an electronic oscillator including a vacuum tube 38. The cathode of this tube is connected to ground and the anode is connected through a load resistor 38 to the positive terminal of a source of unidirectional energy such as the battery 48. There also is provided afrequency-determining circuit 4| which is coupled between the anode and control grid of the tube 31 by condensers 42 and 43, respectively; There also is connected to the grid of the tube a: resistor 44 which serves as a leak circuit to ground. The output, circuit or the oscillator, which is derived from the anode coupled terminal of the frequency-determining circuit 4|. is connected to the tenth stage multiplier electrode ll.

There is coupled to the inductor 33 of the tunedoutput circuit of the multiplier a center-tapped inductor. The respective terminals of the inductor 43 are connected to the control grids of a pair of amplifier tubes 48 and 41. These tubes may be enclosed in the same envelope, if desired, in which case the tube may be a 832 type ,or its equivalent. The cathodes of the tubes 48 nected to' ground. The anodes of the tubes are connected to the respective terminals of a centertapped inductor 49. The midpoint of the inductor 49 is connected to the positive terminal of a source of unidirectional energy such as the battery 5| of which the negative terminal is connected to ground. There is coupled to the inductor 48 an inductor 52, the terminals of which are connected to any suitable output circuit.

There also is provided a signal mixing tube 53 which preferably is a pentode of the SAC] type or its equivalent. The cathode of this tube is connected through a resistor 54 to ground. The screen grid 55 is connected through a re- --sistor 56 to the positive terminal of the battery 40. There also is provided a by-pass condenser 51 which is-.connected between the screen grid and ground. The control grid 58 of the mii linil tube is coupled to a source of 'frame synchronizingsignals 59 and the suppressor grid 6'! of this tube is coupled to a source of line synchronizing signals 62. The anode of the tube 53 is connected through a load resistor 63 to the positive terminal of the battery 40. There also is' provided a connection from the anode of the mixing tube 53 to the midpoint of the amplifier input inductor 45. I

Referring now to the operation of the embodiment of the invention shown in the drawing, assume that the oscillator, including the vacuum tube 38, is adjusted to generate carrier wave energy at a relatively high frequency, for example, of the order of several hundred megacycles. The voltage divider component including the parallelly connected inductor 21 and condenser 29 is tuned to the carrier wave frequency, and the circuit constants are chosen so that the impedance of this component is of sufiiclent magnitude to develop alternating current energy having a substantial peak-to-peak voltage. which, if desired, may be greater than the unidirectional voltage developed in the divider resistor 26. Also, the multiplier output circuit network 32 is tuned to the carrier wave frequency.

In the case of a television signal generator, an optical image of the television subject is pro- Jected onto the photoelectric cathode I in a conventional manner (not shown). There thus is produced an electron stream of anintensity varying in accordance with the light and shade values of the subject. Under the. control of the defleeting elements 5 and 6, successive portions of the electron stream are admitted to the multiplier apparatus housed within the shielding anode '2 through the scanning apertures 4 and I3. The

normal electronaccelerating voltages impressed upon the multiplying electrodes such as III, II and [2 effect the impingement of the admitted. electrons upon successive multiplier stage electrodes At each impact a multiplied number of secondary electrons is produced.

In a conventionally operated multiplier of this type a continuous multiplication of electrons is effected by the eleventh stage multiplier electrode 12. ,Thisresult is produced by maintaining a substantially fixed unidirectional accelerating voltage between the tenth and eleventh stage elements It and I2, respectively. This normal unidirectional electron accelerating voltage lade- .oped in the resistor 26.

rived from the terminals of the voltage divider resistor 26. However, in this case it is noted that the output circuit of the oscillator, including the tube 38, also is coupled between the multiplier electrodes II and i2. The A. C. voltage developed by the oscillator has a peak-to-peak value which is of substantialmagnitude and, in

some instances, may exceedthe D. C. accelerating voltage impressed upon these two electrodes. Thus, periodically at the oscillator frequency, a1- ternate half cycles 01 the alternating current developed in the parallel resonant circuit, including the inductor 21 and the condenser29, oppose in injecting any desired system control signal components. This is effected by coupling the output circuit of the signal mixing tube 53 to the input circuit of the amplifier, including the tubes 46 and polarity the D. C. voltage developed in the re- 1 sistor 26. Consequently, the electron accelerating voltage impressed between the multiplier elec-' carrier wave frequency. I

It is noted that the voltage divider circuit connected to these electrodes is arranged to permit the respective development of the D. C. and AC. voltages at optimum values. including the inductor 21' and the condenser 29, presents a relatively small impedance to the unidirectional current flowing from the battery l5.

through the voltage divider. In this manner a D. C. voltage of the desired magnitude is devel- Also, this resistor is shunted by the condenser 28 so as to shunt the alternating current flowing in the output circuit of the oscillator around the voltage divider resistor. At the same itme the tuned network, including the inductor 21 and the condenser 29,

has a relatively large A'. C. impedance so as to.

enable the development in the network of an v A. C. voltage of the desired magnitude.

As a result of the described type of multiplier operation the gain of the multiplier is alternately varied between predetermined maximum and minimum values. In this manner there is impressed by the collecting electrode i4 upon the network 32 a series of impulses at the frequency of the carrier wave oscillator. The amplitude of these impulses is modulated by the intensity of the electron flow between the electrodes II and I2 during the periods that a flow is permitted. Inasmuch as the electron multiplication effected in' a device of this character is substantially linear, the intensity of the electron flow between the electrodes II and I2 corresponds in magnitude to the light and shade values of the television subject. Therefore, the amplitude modulation of the impulses impressed upon the network 32 is in accordance with the generated signals.

Being tuned to the oscillator frequency, and, therefore, to the frequencyof the impulses derived from'the multiplier, the network 32 functions to develop alternating current energy of substantially sinusoidal wave form having an amplitude modulation corresponding to the generated signals. The iunctioning of this network The tuned circuit.

. trodes -Hand i2 periodically is.modified at the 41. The output circuit voltages of the tube 53 are varied in accordance with the line and frame synchronizing signals impressed upon the input circ'uit of the tube from the respective synchronizing signal sources 62 and 59. These output circuit voltages of the mixing tube 53 are employed .to vary the input'circuit biasing of the amplifier tubes 46 and 41. Thepolarity of the biasing voltages is arranged suitably to develop voltages in the amplifier load inductor 49 of the proper polarity with respect to the video signals also developed in this inductor.

It is seen from the. foregoing description that apparatus in accordance with the present invention has the advantage of employing an lectron multiplier in a modulating system without being limited to the use of relatively low frequency carrier waves. The earlier multiplier stages function conventionally to increase the intelligence signal energy and, consequently, are not subject 7 7 it also is within the scope of the present invention to introduce this energy between the eleventh (or last multiplier stage electrode and the electron collecting electrode. This-and other obvious structures which are equivalent to that described are considered to come within the spirit and scope of certain of the appended claims.

It is apparent that apparatus embodying the instant invention is not limited for use with a television signal generator. Substantially any type of signal generator may be employed to develop an electron stream of intensity. varying in accordance with the signal whereby apparatus in accordance with'the invention may be employed advantageously to generate carrier frequency energy'which is modulated in amplitude in accord ance with the signal.

thus is comparable to the tuned output circuit of a conventional class C amplifier.

The amplitude modulated carrier frequency energy developed in the network 32 is inductive- 1y reproduced in the amplifier input inductor 45 produced by induction in the coil 52 a correspondingly modulated carrier frequency wave for impression upon a suitable utilization circuit.

v of the invention.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those. skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is-aimed in the appended claims to cover all such changes and modifieations as fall within the true spirit and scope What is claimed is: 1. A modulating system comprising, an electron.multiplier having a pair of secondary electron emissive electrodes and an electron collectductor is of the tubes 46 and 41. There isreing electrode, means for supplying to one of said secondary emissive electrodes a stream of electrons varying in intensity in accordance with a first type of signals, means for impressing a uni- In the case where the modulation of the'car- 1g directional electron accelerating potential bei 1 1 tween said secondary emissive electrodes, means for impressing a carrier wave betweensald secondary emissive electrodes to modify periodically the unidirectional accelerating potential between said electrodes at said carrier wave frequency, a

carrier frequency amplifier having an input cirone of said secondary emissive electrodes a stream of electrons varying in intensity in accordance I with video signals, means for impressing unidirectional electron accelerating potentials between pairs of successive secondar emissive electrodes,

means for modifying periodically the unidirectional accelerating potential between two of said secondary emissive electrodes at a carrier freq'uencir. a multiplier output circuit tuned to said carrier frequency coupled to said collecting electrode wherein to develop carrier frequency energy modulated in amplitude in accordance with said video signals, a carrier frequency amplifier having an input circuit coupled to said multiplier output circuit, means responsive to auxiliary system control signals to develop voltages representative ofsaid control signals, means for variqumcy energy modulated in amplitude in accordance with a composite television signal comprising video and auxiliar system control signal components.

3. A television modulating system comprising, an electron multiplier having a plurality of secondary electron emissive electrodes and an electron collecting electrode, means for supplying to the first one of said secondary emissive electrodes a stream of electrons varying in intensity in accordance with video signals, means for impressing unidirectional electron accelerating potentials between pairs of successive secondary emissive electrodes, means including a carrierfrequency oscillator having an output circuit coupled between the last two of said secondary emissive electrodes for decreasing the unidirectional ac- I celerating potential between said last two electrodes at said carrier frequency, a multiplier output circuit including a network tuned to said carrier frequency coupled to said collecting electrode wherein to develop carrier frequency energy modulated in amplitude in accordance with said video signals, a carrier frequency amplifier having an input circuit coupled to said multiplier output circuit, means including a signal mixing tube responsive to auxiliary system control signals to develop voltages representative of said control signals, means for variably biasing the input circuit of said amplifier by and in accordance with said control signal representative voltages, and an output circuit for said amplifier wherein to develop carrier frequency energy modulated in amplitude in accordance with a composite tele ably biasing said amplifier by said control signal so vision signal comprising video and auxiliary sysrepresentative voltages, and an output circuit for said amplifier wherein to develop carrier fretem control signal components.

, MADISON CAWEIN. 

